1. Field of the Invention
The present invention relates to a microminiature electric current fuse which can be mounted on the surface of a printed circuit substrate to cope with microminiaturization technology of a circuit for electronics and high density of electronic elements mounted on the circuit.
2. Description of Prior Art
The following surface-mount type micro sized electric current fuses according to a prior art have been developed. One example is that which has been developed for the purpose of miniaturization, wherein a fusible part composed of a metallic film is formed on an insulating substrate and subsequently the metallic film constituting this fusible part is embedded in a low melting point glass or plastic material. Another example of a micro sized electric current fuse which has been developed at a low cost is constructed such that after the fusible wire has been connected between metallic electrodes by means of bonding or the like, the fusible wire is integrally molded together with a fuse body in such a manner as to be embedded in the plastic material. However, since a surface-mount type micro sized electric current fuse having such a construction as described above has been developed solely with a view to miniaturization and reduction of cost, the fusible part or the fusible wire (hereinafter collectively referred as "fusible part") comes into direct contact with an insulating member such as a plastic material, low melting point glass or the like, resulting in an inability of the fusible part to maintain thermal neutrality and poor pre-arcing time-current characteristics.
More specifically, such a construction whereby the fusible part comes into direct contact with the insulating material requires a large amount of exoergic action since, when Joule heat is generated at the fusible part due to a flow of abnormal current due to circuit failure or the like of an electronic device, such Joule heat is directly absorbed by the insulating material, resulting in a delay in cutting off such an abnormal current and heating of the insulating material itself with which the fusible part is in contact.
Consequently, a more exoergic action than necessary is required for heading the fusible part so that the internal resistance of the fusible element itself has to be increased and as a result, under normal use conditions, the electric loss on the electric circuit caused by the fusible element alone is considerable and an exoergic action of the fuse element has to be large.
Another example of a microminiature electric current fuse which has been developed in order to attain a higher reliability is constructed such that, after electrodes have been disposed in and fixed to a casing, a fusible wire is extended between the electrodes and soldered thereto and after that an opening is tightly closed by means of a lid. However, it has to be pointed out that, according to the method of soldering the fusible wire to the electrodes, the difference in bulging of the solder upon solidification may cause a variation in a distance between the electrodes or the length of the fusible wire. Such a problem of variation has yet to be solved.
In addition, since the above-mentioned prior art miniature or microminiature electric current fuse requires each component to be machined and assembled one by one in a batch-type process and such machining and assembly operations are difficult due to such components being small or production being difficult to execute, costs have remained relatively high.